稀有金属(英文版) 2020,39(10),1202-1209

Microstructure,tensile properties and free bulge formability of Sr-and Y-containing AZ31 alloy sheet produced by twin-roll casting and sequential hot rolling

Peng Jiang Luo-Bin Li Yan-Dong Yu Wang-Ping Wu Zhi-Zhi Wang Yi Zhang Nai-Ming Miao

School of Mechanical Engineering,Changzhou High Technology Research Key Laboratory of Mould Advanced Manufacturing,Changzhou University

Jiangsu Key Laboratory of Green Process Equipment,Changzhou University

School of Materials Science and Engineering,Harbin University of Science and Technology

作者简介：*Peng Jiang,e-mail:penn.jiang@gmail.com;

收稿日期：7 July 2018

基金：financially supported by the National Natural Science Foundation of China (Nos.51705038 and51875053);the Natural Science Foundation of Jiangsu Province of China (No.BK20150268);

Microstructure,tensile properties and free bulge formability of Sr-and Y-containing AZ31 alloy sheet produced by twin-roll casting and sequential hot rolling

Peng Jiang Luo-Bin Li Yan-Dong Yu Wang-Ping Wu Zhi-Zhi Wang Yi Zhang Nai-Ming Miao

School of Mechanical Engineering,Changzhou High Technology Research Key Laboratory of Mould Advanced Manufacturing,Changzhou University

Jiangsu Key Laboratory of Green Process Equipment,Changzhou University

School of Materials Science and Engineering,Harbin University of Science and Technology

Abstract：

The effects of Sr and Y on microstructure,tensile property and free bulge formability of AZ31 alloy sheet produced by twin-roll casting and sequential hot rolling were investigated to improve the mechanical properties and formability of the AZ31 alloy.Sr and Y addition can form Al₄ Sr,Al₂ Y and Al₃ Y phases which can impede dislocation movement and promote dynamic recrystallization during the rolling deformation and decrease the lattice resistance to dislocation motion by decreasing Al solubility in the alloy,resulting in finer grains,lower dislocation density and no twinning generating in the twin-roll casting and sequential hot rolling(TRC-HR) AZ31-1.3 Sr-1.0 Y alloy.The maximum stress and elongation of the alloy increase significantly after adding Sr and Y.The average cavity and grain sizes of the TRC-HR AZ31-1.3 Sr-1.0 Y alloy are smaller,resulting in higher elongation in the alloy.The addition of Y and Sr can effectively improve the free bulge formability and the thickness uniform of the alloy.The Al₄ Sr,Al₂ Y and A13 Y phases can inhibit the grain growth by obstructing dislocation motion or grain boundary slip,resulting in smaller grain size of AZ31-1.3 Sr-1.0 Y alloy bulge parts.

Keyword：

AZ31 alloy; Strontium; Yttrium; Twin-roll casting; Tensile properties; Free bulge formability;

Received： 7 July 2018

1 Introduction

Magnesium alloy with low density,high rigidity and high strength is an attractive and promising structural engineering alloy,which is considered to be“a green engineering material”in the twenty-first century,and has been used in the automobile industry and 3C (computer,communication,and consumer electronics) [ 4] .However,many limitations are associated with Mg alloy compared to other metals,including lower tensile properties (strength and ductility),low creep resistance,poor workability due to its hexagonal structure [ 5, 6, 7] ,which severely restricts the widespread application of Mg alloy.The strengthening of properties can be obtained through proper alloying and processing conditions.Hence,numerous alternative approaches to overcome the problems associated with Mg alloys have been explored by material scientists and engineers in recent years.Grain refinement has been considered as one of the most effective approaches [ 8] .Grain refinement can not only simultaneously improve strength,toughness and ductility,reduce casting defects,such as segregations and porosity,but also eliminate the columnar structure to increase the quality of wrought alloys through improvement in formability [ 9] .The addition of proper elements (such as Ti [ 10, 11, 12] ,Zr [ 13] ,Ca [ 14, 15, 16, 17, 18, 19] ,Si [ 20] ,Cu [ 21] ,Sr [ 15, 16, 18, 22, 23, 24, 25] and RE [ 14, 19, 22, 26, 27, 28, 29] )and thermal-mechanical treatment [ 30, 31, 32] upon solid Mg alloys are two of the effective approaches for grain refinement.Twin-roll casting (TRC) has become increasingly important because of the reduction in production steps in the production of strip compared to conventional technologies,and processing costs are a third to a half as high.TRC sheets have equivalent or superior properties to conventional wrought magnesium alloy sheets [ 33, 34] .

In the previous work,we studied the tensile properties and fracture behavior of AZ31+Sr+Y magnesium alloy sheets at various temperatures and found that the microstructure of the AZ31 alloy was refined and the elongation increased by adding elements Sr and Y,and the fracture behavior of the alloy changed from brittle fracture to ductile fracture with temperature increasing [ 35] .In this work,a combination of the alkaline earth Sr and the rareearth Y addition and twin-roll casting and sequential hot rolling (TRC-HR) treatment was utilized to refine grains and improve the formability of AZ31 alloys.The effects of Sr and Y on microstructure,tensile property,cavities and free bulge formability of the AZ31 alloy sheets produced by TRC-HR technology were investigated,and the mechanism of Sr and Y improving the mechanical properties of AZ31 alloys was in-depth analyzed.

2 Experimental

AZ31 and AZ31-1.3Sr-1.0Y magnesium alloy sheets were used in this work.The detailed compositions of Sr and Y are 1.29 wt%and 1.02 wt%,respectively.The two sheets were prepared by TRC-HR technique (Commonwealth Scientific and Industrial Research Organization,CSIRO).The sheets with the thickness of 3.6 mm were obtained by TRC technique and homogeneously treated at 420℃for4 h,and then the samples were hot-rolled into 1.0 mm in thickness by multiple passes at 420℃and heated for20 min at pass space.

Tensile specimens with 10 mm gauge in length,5 mm in width and 1 mm in thickness were machined from TRC-HR sheets,and the tensile direction was parallel to the rolling direction.Tensile tests were carried out on a tensile machine at the temperature range from room temperature(RT) to 400℃at an initial strain rate of 7×10^-4 s^-1.

Free bulging formability was evaluated by a blow forming method under gas pressure,and the schematic representation of gas bulging die is shown in Fig.1.The blow forming was performed on a press machine equipped with a silicon carbide furnace,which can make the sheet and die combined tightly,and the sheet and die were continuously heated.Nitrogen gas was introduced into a space between the die and sheet through the upper die airway to provide the forming pressure.The sheet and die pre-coated graphite were heated to 300 and 400℃for30 min,respectively.Then,the gas pressure was applied and the pressure was increased by 0.1 MPa for 2-10 min every time.Under gas pressure,the sheet was continuously expanding until its rupture.The final pressure was approximately 0.6-0.7 MPa.

Fig.1 A schematic representation of gas bulging die

An Olympus optical microscope (OM,GX71-6230A)was utilized to observe microstructure,and the samples surfaces were etched in a mixture of nitric acid,acetic acid,ethanol and water.A scanning electronic microscope(SEM,FEI Quanta 200) was used for the fracture morphology observation.Dislocations were examined by a transmission electron microscope (TEM,JEM-2100,JEOL,Japan) operating at an accelerating voltage of 200 kV.0.1-mm-thin samples were cut from TRC-HR alloys to prepare TEM samples,and then the 0.1-mm-thin sample was ground to 50μm by different grit sandpapers.The samples were finally thinned using conventional ion thinning at voltage of 4.4 kV and angle of 6°-8°.

3 Results and discussion

3.1 Microstructures of TRC-HR alloy sheets

Figure 2 shows microstructures in the RD-TD and RD-ND planes of TRC-HR AZ31 sheet (Fig.2a,b) and TRC-HR AZ31-1.3Sr-1.0Y sheet (Fig.2c,d),where RD,ND and TD represent rolling,normal and transverse directions,respectively.The grains are almost equiaxed and nonuniform,indicating that dynamic recrystallization (DRX)occurs during hot rolling of the two sheets.The average grain size and uniformity of the TRC-HR AZ31-1.3Sr-1.0Y sheet are smaller and better than those of the TRC-HR AZ31 sheet,respectively.The grains are slightly elongated in the RD-ND plane of the two sheets due to that the rolling force acts on the RD-TD plane,and the RD-ND plane is a free surface.It should be noticed that only TRC-HR AZ31 sheet has a twinning structure in RD-ND plane(Fig.2b).More precipitates (black particles) appear in TRC-HR AZ31-1.3Sr-1.0Y sheet than in TRC-HR AZ31sheet.Figure 3 shows TEM images of TRC-HR AZ31 and AZ31-1.3Sr-1.0Y sheets.It can be observed that the dislocation density is higher in TRC-HR AZ31 sheet than in TRC-HR AZ3 1-1.3Sr-1.0Y sheet,and twinning is observed in TRC-HR AZ31 sheet (Fig.3a).

Fig.2 OM images in a RD-TD,b RD-ND planes for TRC-HR AZ3l and c RD-TD,d RD-ND planes for AZ31-1.3Sr-1.0Y sheets

In the previous work,it has been reported that Al4Sr,Al2Y and Al₃Y phases coexist with Mg17Al₁₂ in TRC-HR AZ31-1.3Sr-1.0Y sheet [ 35] .The Al₄Sr,Al₂Y and Al₃Y phases can impede dislocation movement and promote DRX during the rolling deformation,leading to grain refinement.Moreover,the high concentration of Al solute would pin the growing boundaries of the new recrystallized nucleates and restricts recrystallization.In contrast,in the alloy with higher Sr and Y concentrations,the lower levels of Al would reduce pinning effect [ 36] .Therefore,the degree of DRX in TRC AZ31-1.3Sr-1.0Y alloy is higher than that in TRC AZ31 alloy during hot rolling,and finer grains and lower dislocation density can be observed in TRC-HR AZ31-1.3Sr-1.0Y sheet.In TRC-HR AZ31 alloy(Fig.2b),the formation of twins is attributed to the high concentration of Al solute inside the grains and the solute drag effect which limits dislocation movement.No twinning can be observed in TRC-HR AZ31-1.3Sr-1.0Y alloy,which can be related to that the positive effect of grain boundary slip (GBS) and the decrease in the lattice resistance (the Al solute drag effect) on dislocation motion is greater than the negative effect of Al-Sr and Al-Y precipitates on dislocation motion.GBS is an important deformation mechanism in AZ31 alloy at high temperature.Grains are finer in TRC-HR AZ31-1.3Sr-1.0Y alloy,increasing the influence of GBS.As a result,dislocation motion is easier in TRC-HR AZ31-1.3Sr-1.0Y alloy,and twining is reduced or limited [ 36, 37] .

Fig.3 TEM images of a TRC-HR AZ31 and b AZ31-1.3Sr-1.0Y sheets

3.2 Tensile properties

The true stress-strain curves of TRC-HR AZ31 and AZ31-1.3Sr-1.0Y sheets obtained by tensile tests at the strain rate of 7×10^-4 s^-1 and various temperatures were reported in previous work [ 35, 38] .The shape of the stressstrain curve changes at 200℃due to that the dynamic recovery occurs in the two sheets.When the temperature further increases (above 200℃),a stable deformation happens because the hardening and the softening reach a dynamic equilibrium.Figure 4 shows the relation curves of temperature versus the maximum stress and elongation of TRC-HR AZ31 and AZ31-1.3Sr-1.0Y sheets.The maximum stress decreases and ductility significantly enhances with temperature increasing,and the superplasticity (ductility beyond 200%) is obtained at 300 and 400⁰C in the two sheets.GBS plays a dominant role during superplastic deformation in the two sheets [ 35, 38] .It is noted that the maximum stress and elongation of the TRC-HR AZ31-1.3Sr-1.0Y sheet are both higher than those of TRC-HR AZ31 sheet,illustrating that the mechanical properties of the alloy increase significantly after adding Sr and Y.It is related to fine-grained strengthening and impeding of particles to dislocation motion and GBS (at high temperature).The Al_4Sr,Al₂Y and Al₃Y phases in TRC-HR AZ31-1.3Sr-1.0Y alloy can impede dislocation motion to enhance the strength,but it is harmful for plasticity.In this work,ductility can be enhanced in the alloy containing Al₄Sr,Al₂Y and Al₃Y phases;a possible reason is due to the higher level of enhancing ductility by refined grains (fine-grained toughening),GBS and weakening lattice resistance,and the lower level of decreasing ductility by particles impeding to dislocation motion and GBS.

Fig.4 Temperature-max stress-elongation curves of TRC-HR AZ31and AZ31-l.3Sr-1.OY sheets

3.3 Cavities of the fracture samples

SEM images of fracture surface of the tensile specimens were reported in Refs. [ 33, 36] .SEM image perhaps cannot reflect the actual cavity morphology of the alloy interior due to the difference between the free surface and alloy interior.The samples cut near fracture were polished,etched and observed by OM to reveal the cavitation in the alloy interior.Figure 5 shows the cavities near the fracture surface of two sheets obtained at the strain rate of7×10^-4 s^-1 and various temperatures.It is observed that serious cavitation occurs in the alloy interior after superplastic deforming and the intergranular cavities are the clear majority.The average cavity size increases significantly with deforming temperature increasing.The cavities enlarge along the tensile direction (vertical),and the boundaries of the intergranular cavities are irregular serrated shape,indicating that the intergranular cavity is caused by GBS [ 39] .The particles in the alloy can obstruct grain growth to reduce grain boundary stress,resulting in the difficulty in the nucleation and growth of cavities.Therefore,the average cavity and grain size of the TRC-HR AZ31-1.3Sr-1.0Y sheet containing fine Al₄Sr,Al₂Y and Al₃Y particles are smaller than those of the TRC-HR AZ31 sheet after superplastic deforming,as shown in Fig.5.The existence of finer cavities is beneficial to GBS to enhance superplastic [ 40] ,resulting in the higher elongation in the TRC-HR AZ31-1.3Sr-1.0Y sheet.

3.4 Free bulge formability and microstructures

Figure 6 shows free bulge forming parts fabricated at 300and 400℃,cutting the dome in half to observe easily.The height-diameter ratios (H/d) of the parts for TRC-HR AZ31 alloy formed at 300 and 400℃are 0.33 and 0.42,respectively,while those for TRC-HR AZ31-1.3Sr-1.0Y alloy are 0.37 and 0.48,respectively.This indicates that the addition of Y and Sr can effectively improve the free bulge formability of the alloy.

The thinning rates of formed parts are calculated by measuring the thickness before and after forming,and the measured points are 0°,30°,60°and 90°positions on formed parts,as shown in Fig.7.Figure 8 shows the thinning rate curves along the centerline of the domes formed at 300 and 400℃.It's observed clearly that the thickness of all formed parts is reduced with the bulging part height increasing.The thinnest position occurs at the dome apex,and the lowest thinning rate occurs at the clamped periphery of all formed parts.The corresponding thinning in biaxially formed parts is usually due to strainrate gradients caused by local stress differences [ 41] .Therefore,the strain-rate gradient during forming a domeresults in the higher thinning rate at the dome apex.Theminimum thickness and thinning rate of AZ31 and AZ31-1.3Sr-1.0Y alloy parts formed at 300℃are0.58 mm,27.5%,and 0.56 mm,29.5%,respectively,while those of AZ31 and AZ31-13Sr-1.0Y alloy parts formed at400℃are 0.34 mm,57.5%,and 0.28 mm,65.3%,respectively.This fact indicates that Sr and Y addition and higher temperature are beneficial to the superplasticity of AZ31 alloy sheet and enhance the free bulge formability.Thickness thinning rates for two sheets formed at a higher temperature of 400℃are more uniform,which is due to more uniform strain-rate distribution at higher temperature.It should also be noted that the thickness thinning rate of AZ31-1.3Sr-1.0Y alloy parts is more uniform than that of AZ31 alloy parts for forming at 300 and 400℃,showing that Sr and Y addition can improve the thickness uniform.

Fig.5 OM images of cavities of two alloys obtained at strain rate of 7×10^-4 s^-1 and various temperatures:a 300℃,b 400℃for AZ31 and c 300℃,d 400℃for AZ31-1.3Sr-1.0Y

Fig.6 Hemispherical domes (in half) formed under various conditions

Fig.7 Measurement points sketch of hemispherical dome thickness

Fig.8 Thinning rate for free bulge forming parts of two alloys at 300and 400℃

Fig.9 Apex OM images of AZ31 domes formed at a 300 and c 400℃and of AZ31-1.3Sr-1.OY domes formed at b 300 and d 400℃

Figure 9 shows apex microstructures of the four domes formed at 300 and 400℃.It is clearly observed that the grain size of the dome apex is larger than that of TRC-HR sheet,and some small grains are found to surround some large ones,which indicates that DRX and grain growth occur during free bulge forming of the two sheets.The apex grain size of the domes formed at a higher temperature of 400℃is larger due to the more rapid rate of grain growth at higher temperature.It should be noted that the grain size of the AZ31-1.3Sr-1.0Y alloy parts is finer than that of the AZ31 alloy parts.This is attributed to the Al₄Sr,Al₂Y and Al₃Y phases which inhibit the grain growth by obstructing dislocation motion or GBS.

4 Conclusion

In this work,the effects of Sr and Y on microstructure,tensile property,cavities and free bulge formability of AZ31 alloy sheet produced by twin-roll casting and sequential hot rolling were investigated and the mechanism of Sr and Y improving the mechanical properties of AZ31alloys was in-depth analyzed.Experimental results show that Y and Sr can effectively improve the mechanical properties and formability of the AZ31 alloy.Finer grains and lower dislocation density can be observed in the TRC-HR AZ31-1.3Sr-1.0Y sheet.Sr and Y addition can refine grains,increase GBS influence,and decrease Al solute drag effect to dislocation motion,whose positive effect on dislocation motion is greater than the negative effect of Al-Sr and Al-Y precipitates on dislocation motion,resulting in no twinning generating in the TRC-HR AZ31-1.3Sr-1.0Y alloy.

The maximum stress and elongation of the alloy increase significantly after adding Sr and Y.It is related to refined grain and impeding of particles to dislocation motion and GBS.The intergranular cavity is caused by GBS.The average cavity and grain sizes of the TRC-HR AZ31-1.3Sr-1.0Y alloy are smaller.Ductility can be enhanced in the alloy containing Sr and Y;a possible reason is due to the higher level of enhancing ductility by refined grains (fine-grained toughening),GBS and weakening lattice resistance,and the lower level of decreasing ductility by particles impeding to dislocation motion and GBS.Y and Sr can effectively improve the free bulge formability and the thickness uniform of the alloy bulge parts.

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